The present invention relates to an apparatus for pulling a single crystal and, more particularly, to an apparatus for pulling a single crystal with which a silicon single crystal used as a semiconductor material is pulled in its low defect density state.
There are various methods for growing a single crystal, and one of them is a single crystal growth method called the Czochralski method (hereinafter, referred to as the CZ method). FIG. 1 is a diagrammatic sectional view of an apparatus for pulling a single crystal used for the CZ method, and in the figure, reference numeral 1 represents a crucible.
The crucible 1 comprises a bottomed cylindrical quartz crucible 1a and a bottomed cylindrical graphite crucible 1b fitted on the outer side of the quartz crucible 1a. The crucible 1 is supported with a support shaft 8 which rotates in the direction shown by the arrow in the figure at a prescribed speed. A heater 2 of a resistance heating type and a heat insulating mold 7 arranged around the heater 2 are concentrically arranged around the crucible 1. The crucible 1 is charged with a melt 3 of a material for forming a crystal which is melted by the heater 2. On the central axis of the crucible 1, a pulling axis 4 made of a pulling rod or wire is suspended, and at the front thereof, a seed crystal 5 is held by a holder 4a. These parts are arranged within a water cooled type chamber 9 wherein pressure can be controlled.
A method for pulling a single crystal 6 using the above-described apparatus for pulling a single crystal is described below. By reducing the pressure in the chamber 9 and introducing an inert gas thereto, the atmosphere in the chamber 9 is made to be an inert gas atmosphere under reduced pressure. Then, the material for forming a crystal is melted by the heater 2 and is left standing for a period of time so as to release gas in the melt 3 sufficiently.
While the pulling axis 4 is rotated on the same axis in the reverse direction of the support shaft 8 at a prescribed speed, the seed crystal 5 held by the holder 4a is caused to descend and is brought into contact with the melt 3 so as to make the seed crystal 5 partially melt into the melt 3. Then, the single crystal 6 is grown at the lower end of the seed crystal 5.
In growing the single crystal 6, the seed-narrowing (6a) is conducted in order to make the single crystal 6 dislocation-free. Then, a shoulder portion 6b is grown so as to obtain the single crystal 6 having a required diameter in a body portion 6c. When the diameter of the single crystal 6 becomes a required one, the shoulder formation is finished. While the diameter is kept uniform, the body portion 6c is grown. After the body portion 6c is grown to a prescribed length, the tail-narrowing is conducted in order to separate the single crystal 6 from the melt 3 in the dislocation-free state. Then, the single crystal 6 separated from the melt 3 is cooled under prescribed conditions. Wafers manufactured by processing the single crystal 6 thus obtained are used as a substrate material of various semiconductor devices.
In the silicon single crystal pulled through the above steps, defects called infrared scatterers (COP and FPD), dislocation clusters or the like sometimes exist. These defects are not newly formed within the crystal by the later heat treatment. They are already formed during crystal pulling, which are also called grown-in defects.
FIG. 2 is a diagram showing the general relation between the pulling speeds during single crystal growth and occurrence positions of crystal defects. As is shown in FIG. 2, infrared scatterers 21 among the grown-in defects observed in the evaluation after crystal growth are detected inside a ring region of oxidation-induced stacking fault (OSF) 22, a kind of thermally-induced defect. Defects called dislocation clusters 24 among the grown-in defects are detected outside the ring region 22. And a defect-free region 23 exists close to the outside of the ring region (R-OSF) 22. The occurrence region of the ring region (R-OSF) 22 depends on the pulling speed during single crystal growth. As the pulling speed is made lower, the region wherein the ring region (R-OSF) 22 appears shrinks inward from the outer side of the crystal.
The above OSF is an interstitial dislocation loop which occurs during oxidative heat treatment. When the OSF is generated and grows on a wafer surface which is an active region of a device, it causes a leakage current, so that it becomes a defect which deteriorates device properties. Therefore, hitherto, the high-density region of OSF is push out toward the perimeter of the crystal by controlling the position of the R-OSF so as to move toward the perimeter thereof during single crystal growth.
However, recently, the adverse effects of the OSF on devices are controlled since the device manufacturing process is conducted at lower temperatures and a crystal contains less oxygen. Therefore, the OSF is inconsiderable as a factor which deteriorates the device properties. On the other hand, the infrared scatterers among the grown-in defects are a factor which deteriorates the time zero dielectric breakdown, and the dislocation clusters are a factor which remarkably deteriorates the device properties. Recently it is an important problem to reduce the density of those grown-in defects within a crystal.
Accordingly, it is attempted to obtain high-quality devices by utilizing the region wherein almost no defects to deteriorate the device properties are detected, or, the defect-free region existing close to the inside or outside of the ring region (R-OSF) 22. But since the above defect-free region is a very limited region, it is difficult to utilize it effectively. In order to deal with these problems, some methods were proposed.
For example, in Japanese Patent Laid-Open No. 08-330316, it is disclosed that a crystal wherein only the outside region of a ring region (R-OSF) 22 spreads all over the surface thereof can be grown by improving the crystal growth conditions so as to generate no dislocation clusters. However, there is a possibility to achieve this only with a very limited crystal growth condition, or, only by controlling the pulling speed within a very limited range to a certain temperature gradient. It is an extremely severe condition in the silicon single crystal growth wherein a crystal will have a larger and larger diameter and mass production thereof is required.
In Japanese Patent Laid-Open No. 07-257991, and Journal of Crystal Growth, 151 (1995) pp.273-277, it is disclosed that the outside region of a R-OSF can be generated by making the temperature gradient in the pulling axis direction large and pulling a single crystal at a high speed so as to annihilate the R-OSF on the inside of the crystal. However, reduction of grown-in defects within a crystal plane is not considered at all. Even if the R-OSF is caused to shrink inward, dislocation clusters exist in the outside region of the R-OSF as before. Since the dislocation clusters greatly deteriorate the device properties, it cannot be said that high-quality wafers are provided.
The present invention was developed in order to solve the above problems, and it is an object to provide an apparatus for pulling a single crystal with which a single crystal having a low density of grown-in defects called infrared scatterers, dislocation clusters or the like can be grown.
The present inventors examined the occurrence situation of dislocation clusters to the R-OSF occurrence position within a single crystal grown with conventional conditions or a single crystal wafer and the width thereof. In order to make clear the R-OSF occurrence position within a wafer surface, the distance from the center of the crystal (wafer) to the perimeter (or, the crystal radius) is represented by R, while the R-OSF occurrence position in the radial direction within the crystal is represented by r. For example, when the R-OSF occurs in the center of the crystal, r equals 0. When it occurs on the perimeter of the crystal, r equals R. Here, the R-OSF occurrence position is the position of the inner diameter thereof.
FIG. 3 is a graph indicating the occurrence situation of dislocation clusters based on the relationship between the width of an R-OSF in a single crystal grown with conventional conditions and occurrence position thereof. Here, the width (%) of the R-OSF is shown by the proportion of the length to the radius of the grown crystal. In FIG. 4, the defect distribution within a wafer surface where the R-OSF occurrence position r is (2/3) R is diagrammatically shown.
From the results shown in FIG. 3, it is found that the width of the R-OSF under the conventional growth conditions is 8% of the crystal radius at a maximum, and that dislocation clusters are generated without exception when the R-OSF occurrence position r is (2/3)R or less. In other words, when the width of the R-OSF becomes larger, there is supposedly a possibility that a crystal having no occurrence of dislocation clusters can be grown even if the R-OSF occurrence position r is made smaller (in order to make the occurrence region of infrared scatterers smaller), for example, rxe2x89xa6(2/3)R.
As is described below in detail in the BEST MODE FOR CARRYING OUT THE INVENTION, FIG. 5 is a graph indicating the occurrence situation of dislocation clusters based on the relationship between the width of an R-OSF and occurrence position thereof in a crystal having a diameter of 8 inches obtained by carrying out an embodiment of the present invention. This proves the above supposition.
FIG. 6 is a diagram showing the relationship between the concentration distribution of vacancy taken inside a plane of a crystal grown with conventional conditions and width of a R-OSF to be generated. In the figure, the axis of ordinates shows the vacancy concentration, while the axis of abscissas shows the position within the crystal plane.
As is shown in FIG. 6, the width of the R-OSF 22 within a conventional crystal plane is 8% or less of the crystal radius. This suggests that the region where the R-OSF 22 occurs is only the region corresponding with the part having vacancy concentrations 31 within a limited range, and that the range corresponding with the vacancy concentrations 31 is within the range of 8% of the crystal radius. In the figure, reference numeral 32 represents vacancy concentrations in the range to be a defect-free region.
In a generally pulled crystal, the temperature gradient varies in the pulling axis direction within a plane. As is shown in FIG. 7(a), since the temperature in the outer region becomes low sooner, the temperature gradient (G) becomes larger in the outer region. When the temperature gradient becomes larger, the amount of vacancy taken inside the crystal which diffuses toward the solid-liquid interface in the pulling axis direction and disappears becomes larger and the concentration of the vacancy kept in the crystal becomes lower. Since the amount of vacancy in the outer region of the crystal which diffuses outward in the crystal radial direction is larger than that in the crystal center portion, the vacancy concentration in the crystal outer region is lowered more easily. Thus, the concentration of vacancy taken inside the crystal plane is not uniform. The concentration lowers nearer the crystal perimeter.
Under the conventional growth conditions, since the vacancy concentration within the crystal plane is not uniform and greatly lowers nearer the crystal perimeter, the region wherein the vacancy concentrations in which the R-OSF occurs correspond with the vacancy concentrations within the crystal plane is small, which is within the range of 8% of the crystal radius. As a result, hitherto, the width of the R-OSF is held down to 8% or less of the radius of the grown crystal.
It is ascertained that the R-OSF occurrence region is determined by the crystal pulling speed and the temperature region of the hottest portion (the melting point to 1200xc2x0 C.) during crystal pulling, and is affected by the thermal history in the hottest portion during pulling.
From the above contents, the present inventors found out that the width of the R-OSF 22 can be increased by controlling the thermal history of the hottest portion (the melting point to 1200xc2x0 C.) of the crystal during crystal pulling so as to make the then temperature gradient in the outer region within the crystal plane equivalent to (see FIG. 7(b)), or less than that in the center portion (see FIG. 7(c)) to enlarge the region of the vacancy concentration range wherein the R-OSF is generated (see FIG. 8). FIGS. 7(a)-7(c) are diagrams showing the relationship between the position within a crystal plane and temperature distribution. FIG. 8 is a diagram showing the relationship between the concentration distribution of vacancy taken inside a plane of a grown crystal and width of an R-OSF to be generated when the region of the vacancy concentration range wherein the R-OSF is generated is enlarged.
The main heat flows on the side surface of the crystal in the hottest portion thereof are heat input from the heater and heat radiation to the chamber. Therefore, in order to make the temperature gradient in the crystal outer region smaller, it is necessary to make the heat input from the heater larger, to make the heat radiation to the chamber smaller, or to conduct both of these. In the present invention, concerning the heat flows, the temperature gradient is regulated mainly by making the heat radiation to the chamber smaller.
In accordance with a first aspect of the present invention, provided is an apparatus for pulling a single crystal having a crucible to be charged with a melt, a heater arranged around the crucible, a straightening vane in the shape of a side surface of an inverted truncated cone or a cylinder surrounding a pulled single crystal and the like, wherein the lower end portion of the straightening vane is located above the surface of the melt to be filled in the crucible, characterized by a ring-shaped heat shield plate being fitted to, or being arranged off the inner or outer wall surface of the straightening vane at a position higher than the lower end portion of the straightening vane.
Using this apparatus for pulling a single crystal, the heat radiation to the chamber from the outer region of the single crystal in the hottest portion can be reduced, so that the temperature gradient in the outer region of the pulled single crystal can be made equivalent to or less than that in the crystal center portion. By enlarging the region of the vacancy concentration range wherein an R-OSF is generated, the width of the R-OSF can be increased. Therefore, by shrinking the occurrence position of the R-OSF and increasing the width thereof, a single crystal having a low defect density can be grown. In addition, since the ring-shaped heat shield plate can be fitted by simply mounting it on the straightening vane, the heat shield plate can be easily arranged.
In accordance with a second aspect of the present invention, provided is an apparatus for pulling a single crystal according to the first aspect of the present invention characterized by the distance between the melt surface and the bottom surface of the heat shield plate being within the range of 30 mm-200 mm.
Using the apparatus for pulling a single crystal in accordance with the second aspect, inhibiting the radiant heat from diverging upward from the side surface of a pulled single crystal located in the vicinity of the melt surface and inhibiting the radiant heat from diverging upward from the melt surface and the upper portion of the crucible can be effectively conducted with the heat shield plate.
In accordance with a third aspect of the present invention, provided is an apparatus for pulling a single crystal according to the first or second aspects of present invention characterized by a heat insulator being embedded in the upper portion of the straightening vane above the position where the heat shield plate is arranged in the apparatus.
Using the apparatus for pulling a single crystal in accordance with third aspect, the whole temperature gradient in the pulling axis direction can be made larger by the heat insulator embedded in the straightening vane. When the temperature gradient in the pulling axis direction in the whole single crystal becomes larger, the diffusion rate of the vacancy taken inside the crystal toward the solid-liquid interface becomes higher, so that it becomes difficult to secure the region wherein the vacancy concentration in which an R-OSF occurs and the vacancy concentration within the crystal plane are coincident with each other at usual pulling speeds. In order to hold the diffusion rate down, the pulling speed can be increased. Therefore, the production efficiency of single crystals can be improved. In order to maintain the heat input from the heater large, the heat insulator is not embedded in the lower portion of the straightening vane. Here, by making the lower portion of the straightening vane of quartz, the heat input from the heater can be further increased.
In accordance with a fourth aspect of the present invention, provided is an apparatus for pulling a single crystal according to any of the first to third aspects of the present invention characterized by the inner diameter of the heat shield plate being larger than the outer diameter of a single crystal to be pulled when the heat shield plate is on the inner surface side of the straightening vane.
Using the apparatus for pulling a single crystal in accordance with the fourth aspect, the heat shield plate can be arranged in a desired position without being an obstacle to the single crystal.
In accordance with a fifth aspect of the present invention, provided is an apparatus for pulling a single crystal according to any of the first to third aspects of the present invention characterized by the outer diameter of the heat shield plate being smaller than the inner diameter of the crucible to be charged with a melt when the heat shield plate is on the outer surface side of the straightening vane.
Using the apparatus for pulling a single crystal in accordance with the fifth aspect, the heat shield plate can be arranged in a desired position without being an obstacle to the crucible.
In accordance with a sixth aspect of the present invention, provided is an apparatus for pulling a single crystal according to any of the first to fifth aspects of the present invention characterized by having a cooling tube surrounding a single crystal pulled above the straightening vane.
Using the apparatus for pulling a single crystal in accordance with the sixth aspect, the whole temperature gradient in the pulling axis direction of the single crystal can be made larger by arranging the cooling tube in a prescribed position. When the temperature gradient in the whole single crystal becomes larger, the diffusion rate of the vacancy taken inside the crystal toward the solid-liquid interface becomes higher. In order to secure the region wherein the vacancy concentration in which an R-OSF occurs and the vacancy concentration within the crystal plane are coincident with each other so as to hold the diffusion rate down, the pulling speed can be increased. Therefore, the production efficiency of single crystals can be further improved.
In accordance with a seventh aspect of the present invention, provided is an apparatus for pulling a single crystal according to any of the first to sixth aspects of the present invention characterized by having the lower portion of the straightening vane widen toward the lower end, and providing a flection thereof located below the fitting position of the heat shield plate.
Using the apparatus for pulling a single crystal in accordance with the seventh aspect, since the inside of the lower portion of the straightening vane is effectively heated from the melt surface which is a rather hot portion in the crucible, the heat input to the outer surface of the single crystal facing the inside of the lower portion of the straightening vane can be increased. As a result, more certainly, the temperature gradient in the outer region of the single crystal can be made equivalent to or less than that in the center portion thereof.
In accordance with an eighth aspect of the present invention, provided is an apparatus for pulling a single crystal according to the seventh aspect of the present invention characterized by the lower end portion of the main body of the cooling tube being located above the ring-shaped main body of the heat shield unit or the heat shield plate.
Using the apparatus for pulling a single crystal in accordance with the eight aspect, by locating the lower end portion of the main body of the cooling tube above the ring-shaped main body of the heat shield unit or the heat shield plate, the temperature gradient in the pulling axis direction of the single crystal is made still larger as a whole, so that the pulling speed can be further increased.
In accordance with a ninth aspect of the present invention, provided is an apparatus for pulling a single crystal according to any of the first to eighth aspects of the present invention characterized by the distance between the ring-shaped main body of the heat shield unit or the heat shield plate and the melt surface ranging within values resulting from the diameter of a single crystal to the pulled being multiplied by 0.2-1.5.
Using the apparatus for pulling a single crystal in accordance with the ninth aspect, since the ring-shaped main body of the heat shield unit or the heat shield plate can be arranged at the position where the crystal temperature is 1000-1300xc2x0 C., the temperature gradient in the hottest portion of the single crystal can be more easily controlled.
In accordance with a tenth aspect of the present invention, provided is an apparatus for pulling a single crystal according to any of the first to fifth aspects of the present invention characterized by the ring width of the ring-shaped main body of the heat shield unit or the heat shield plate being 10 mm or more.
When the ring width of the ring-shaped main body of the heat shield unit or the heat shield plate is less than 10 mm, the heat shield force is weak, so that the heat shield effect cannot be expected. But using the apparatus for pulling a single crystal in accordance with the tenth aspect, the heat shield effect can be sufficiently secured.
In accordance with an eleventh aspect of the present invention, provided is an apparatus for pulling a single crystal according to any of the first to tenth aspects of the present invention characterized by the thickness of the ring-shaped main body of the heat shield unit or the heat shield plate being within a range of 2 mm-150 mm.
Using the apparatus for pulling a single crystal in accordance with the eleventh aspect, the strength of the ring-shaped main body of the heat shield unit or the heat shield plate can be secured by making the thickness thereof 2 mm or more. Even when the thickness thereof exceeds 150 mm, the heat shield effect is not so different. Accordingly, by making the thickness thereof 150 mm or less, the cost can be inhibited.